The present invention relates to image-forming apparatus employing a clock-generating circuit, and specifically relates to an image-forming apparatus employing a clock-generating circuit capable of adjusting a number of reference clock-pulses at a predetermined number within a predetermined time interval.
In an image forming apparatus, an image is formed on an image-bearing member by scanning the image-bearing member, which is rotating in the direction of sub-scanning by a laser beam, which is modulated in accordance with image data in the direction of main scanning. In this case, on the basis of reference clock-pulses called dot clock pulses, the laser beam is modulated by the image data.
Accordingly, in accordance with the predetermined number of the dot clock pulses, it is necessary to generate the dot clock pulses in order that the length of the image to be formed on the image-bearing member in a main scanning direction should be always kept constant.
Further, in recent years it has been developed a color image forming apparatus equipped with a plurality of units comprising means for charging, exposure, and development respectively in the vicinity of the image-bearing member, on which a color toner image are formed during one revolution of it, and transferring the images altogether onto a recording paper at a time. On the other hand, it has been also developed a color image forming apparatus equipped with a plurality of image-bearing members in the vicinity of an intermediate transfer member and equipped with means for charging, exposure, development, and transfer around the image-bearing members respectively, transferring the toner images having been formed on the respective image-bearing members sequentially onto the intermediate transfer member, and transferring the color toner images born on the intermediate transfer member altogether onto the recording paper at a time.
In an image forming apparatus of the former type, the length of the image, formed on the image-bearing member in accordance with the predetermined number of the dot clock pulses, sometimes, fluctuates due to variations of the rotation number of the polygonal mirror performing the main scanning operation, the aberration of the optical system, etc.
On the other hand, in an image forming apparatus like the latter one, which employs a plurality of exposing means to form color toner images on the image-bearing member or the intermediate transfer member, owing to the dispersion of the characteristics of the optical systems such as the polygon mirror and a lens in the respective exposing means, the dispersion of the length of image formed on the image-bearing member in the main scanning direction is produced among the respective exposing means, resulting in producing color deviations.
In such cases as described in the above, it is desirable that the rise timing (phase) and the frequency of the dot clock pulses can be adjusted in a fine mode.
For an abovementioned circuit, which makes it possible to adjust the phase and the frequency, a VCXO (Voltage Controlled Xtal (crystal) Oscillator), a DDS (Digital Direct Synthesizer), etc. have been available.
Although both the VCXO and the DDS are sufficient in accuracy, the VCXO is unsuitable for an application of generating the dot clock pulses due to its narrow adjustable range, while the DDS is also unsuitable for such the application, since the DDS circuit is very expensive and has a difficulty in generating high-frequency dot clock pulses.
Further, when a conventional PLL (Phase Locked Loop) is employed for adjusting clock-pulses in a fine mode, the time constant of the PLL should be set at a large value due to a large number of a dividing ratio, resulting in a problem of long-time clock jitters depending on a frequency-stability of its VCO (Voltage Controlled Oscillator). Incidentally, the term of the abovementioned “long-time clock jitters” is defined as a phenomena in which the frequency of the pulses generated by the VCO gradually deviates within a time interval from feedback to feedback due to the large time constant of the PLL.
Further, in another patent application, the present inventors already disclosed a technology for generating dot clock pulses having a predetermined number of pulses within a predetermined time interval in a constant exposing range of a writing means, in which the period of each clock-pulse is slightly increased or reduced by changing a selection of a plurality of delayed clock pulses, which are generated by delaying clock-pulses outputted from a reference oscillator in slightly different delay times. The disclosed technology allows the apparatus to keep the image length of the scanning line in its main-scanning direction constant.
When employing the abovementioned technology, however, a fluctuation of the clock-period (hereinafter, referred to as a short-time jitter or a jitter) occurs at a point where the period of each clock-pulse is slightly increased or reduced (a time-point at which the selection of the delayed clock pulses is changed), resulting in either a stepwise increase or a stepwise decrease of the period of the dot clock pulses within the time-interval of one scanning line, and resulting in a problem of adversely affecting the image-formation.
Further, in a color image-forming apparatus, the exposing widths on the photoreceptor element are apt to vary with the performance variations of the exposing units each of which corresponds to each of primary colors, the assembling-accuracy variations of the exposing units, etc. Therefore, it is necessary for the writing means for each color to keep a predetermined number of dot clock pulses within a constant exposing range. In addition, it is necessary for a duplex image-forming apparatus to shrink the exposing range for a reverse side of a recording material (recording paper), since the recording material shrinks due to the fixing heat applied for fixing an image previously formed on an obverse side of the recording material.
In the above case, although the both sides of the image can be aligned by keeping a predetermined number of dot clock pulses within a constant exposing range, a color mottling would be generated in the image due to jitters occurred at different points when changing a selection of a plurality of delayed clock pulses at the different points in each color, and/or a density unevenness would be generated in the image since the selected dot clock pulses are used for the pulse width modulation of the image data. As a result, such the color mottling and/or the density unevenness have/has adversely affected to the quality of the reproduced image.